In recent years, chargeable and dischargeable lithium-ion secondary batteries (hereinafter, simply referred to as secondary batteries) are used for drive power source of vehicles, such as hybrid cars and electric cars. When such secondary batteries are quickly charged or regenerative current is used for charge in vehicles mounted with secondary batteries, such as hybrid cars and electric cars, a large charging current of 5 C or 10 C, for example, is occasionally applied.
In a case where internal resistance of a secondary battery becomes high due to aging in the secondary battery, even if the same level of the charging currents are applied to the secondary battery, an inter-terminal voltage becomes higher than that at an initial stage (before deterioration, before the increase in the internal resistance) due to the increase in the internal resistance of the secondary battery. On the contrary, in a case where a maximum inter-terminal voltage of the secondary battery is set to a constant value to charge the battery, the larger the value of the internal resistance of the secondary battery is, the higher the inter-terminal voltage becomes. The inter-terminal voltage reaches the maximum value more quickly, and thus the charging cannot be further carried out. For this reason, in the secondary battery whose internal resistance increases, an electric quantity (charge quantity) by which the secondary battery can be charged is reduced more than a time point where the internal resistance is low.
On the contrary, Patent Document 1 discloses a method for charging a lithium-ion secondary battery comprising a step of detecting an internal resistance on charging a lithium-ion secondary battery, and a step of finally charging the secondary battery by performing constant-current charge with a final charging current and performing constant-voltage charge with a final charging voltage. In this technique, the final charging voltage at the final charging step is set to a value obtained by adding a product of the internal resistance and the final charging current of the secondary battery to the set voltage of the secondary battery. For this reason, this method enables a secondary battery to be charged up to the set voltage regardless of the level of the internal resistance. Therefore, according to this technique of Patent Document 1, a secondary battery that is deteriorated and has increased internal resistance can also be sufficiently charged.